Generate prompts/README.md as the directory's entry point #178

Parent

#176

What to build

Give prompts/ an entry point, so an engineer evaluating thirdshift can see everything the agents get from one page.

The golden-file test also generates prompts/README.md, containing:

• the generated-file note
• a short intro using the term Session prompt
• the claude command line for a new session and for a Resume, rendered from the same arguments function as the Prompts and skills page, with the page's placeholders
• a table of the nine Session prompts (title, when it is sent, link to its file)
• a pointer to skills/ for the Factory skills.

It is covered by the same missing, changed and stale-file checks and the same UPDATE_PROMPTS regeneration as the prompt files. The top-level README.md mentions prompts/ next to where it mentions skills/.

Acceptance criteria

• prompts/README.md is generated with the intro, both claude command lines, the table linking all nine prompt files, and the pointer to skills/
• Changing the claude arguments or a prompt's title or "when" sentence makes cargo test fail with a diff of prompts/README.md until regenerated
• Every link in the table resolves to a file in prompts/
• The top-level README.md mentions prompts/ next to skills/

Blocked by

• Generate one prompts/ file per Session prompt, checked by the golden-file test #177